In her latest film, “Young Adult,” screenwriter Diablo Cody has given Charlize Theron a whole new kind of monster to play — the ultimate narcissistic mean girl. The character is Mavis Gary, a stunningly beautiful but depressed 37-year-old divorced alcoholic who still revels in the glory of her high school days and plots to win back her old flame, now married with a newborn child.

Cody, 33, was born Brook Busey in Chicago and now lives in Los Angeles with her husband, Daniel Maurio, and their 18-month-old son Marcello. Cody burst into films in 2007 with her Oscar-winning script for “Juno,” directed by Jason Reitman, who also helmed “Young Adult.” She talked to The Post about self-obsession, addiction to reality TV and leopard-print pedicures.

Mavis is a brilliant meditation on modern narcissism. On a scale of 1-to-10, 10 being Mavis, how narcissistic are you?

I actually took the NPI a couple years ago — the Narcissistic Personality Inventory — and I was surprisingly low. Aside from that data, it’s a hard question. I think anybody who’s in the public eye probably has some issues with self-love. So I’m going to go with a 7.

The movie is veryspecific in its obsession with the ’90s. The first song is played over and over. Did you suggest it?

Yeah, the Teenage Fan Club song [“The Concept”] that’s featured prominently in the movie was one of my favorites. I put it in the script, and I was really fortunate that Jason kept it in there.

Mavis watches a lot of reality TV. What about you?

I watch tons of it. It’s such a cheap thrill. It connects with me the same way I enjoy eating junk food, you know, or listening to Ke$ha. For me, it’s relaxing. My husband likes to play “Call of Duty,” and I like to watch reality TV, because your brain can completely shut down in those situations.

Do you think watching reality shows is a way to stay in high school?

Oh my God, absolutely. It’s like you’re still talking about the popular girl. Or you’re still talking about the breakup that went down in the hallway.

How has having your son changed you?

It changed me personally, although it didn’t really have an effect on my writing the way I thought it would. I thought I was going to be suddenly writing screenplays about families and parenthood. Those subjects are still boring to me.

Were you at all like Jennifer Garner’s character in “Juno” — trying to be the perfect mom and painting the nursery different shades of yellow?

My baby’s room is still in total disarray. I still haven’t readied the nursery, and he’s 16 months old.

Mavis gets tons of what appear to be painful mani-pedis. What was the last one you got?

Actually, the last pedicure I got was, I think, leopard print, which is pretty flamboyant.

Mavis’ life seems fantastic on paper, but she’s very unhappy. Have you ever felt like that?

Yeah, I’ve experienced that for sure. I’ve had tremendous difficulty enjoying my success. I think the trauma has outweighed the joy a little bit. Which is a shame, but I’m trying to cut that out.